1. Field of the Invention
The present invention relates to an optical pickup apparatus that performs an operation of reading a signal recorded in an optical disc.
2. Description of the Related Art
An optical disc device has been widespread, which is capable of performing a signal reading operation by applying laser light emitted from an optical pickup apparatus to a signal recording layer provided in the optical disc.
An operation is performed of reading a signal recorded in the signal recording layer by the optical pickup apparatus by applying the laser light emitted from a laser diode to the signal recording layer and by detecting a change in the laser light reflected from the signal recording layer by a photodetector.
In order to read a signal recorded in the signal recording layer with the laser light, it is necessary to accurately perform a focusing control operation of focusing the laser light to the signal recording layer and a tracking control operation of making the laser light to follow a signal track provided in a spiral state in the signal recording layer.
While there are various methods for performing such a focusing control operation, an astigmatism method using generation of astigmatism is employed in general. While there are also various tracking control methods, a three-beam method using a main beam and two sub beams are generally employed.
Moreover, in order to perform an accurate focusing control operation, a differential astigmatism method using not only a main beam but also a sub beam has recently been employed. In the differential astigmatism method used for the focusing control operation and the three-beam method used for the tracking control operation, a photodetector is provided including two light receiving portions for sub beams to which two sub beams are applied, respectively, and a light receiving portion for main beam to which a main beam is applied is provided. A focus error signal and a tracking error signal are generated from signals obtained by the photodetector, to perform the control operations. Since such an art is well-known, the description will be omitted.
In order to accurately perform the operation of reading a signal recorded in the optical disc and an operation of recording a signal in the optical disc with the optical pickup apparatus, it is an indispensable condition to accurately perform the above-mentioned focusing control operation and the tracking control operation. In order to accurately perform the focusing control operation and the tracking control operation, fixed positions of components making up the optical pickup apparatus or particularly of optical components making up an optical path of the laser light are required to be accurate.
As an element for changing and controlling the optical path of the laser light, an optical component called polarization beam splitter is in widespread use, and the polarization beam splitter is so made up that two prisms are opposed and bonded to each other in such a manner as to sandwich therebetween a control film for selectively controlling transmission and reflection. The polarization beam splitter is provided so as to guide the laser light emitted from the laser diode in a direction of an objective lens provided so as to be focused on the signal recording layer included in the optical disc and so as to guide return light, which is laser light reflected from the signal recording layer, to the photodetector. Therefore, if the mounting position of the polarization beam splitter is not accurate, the above-mentioned control operations in the optical pickup apparatus can not accurately be performed.
An art has been developed for fixing the polarization beam splitter included in the optical pickup apparatus at an accurate position (See Japanese Patent Laid-Open Publication No. 2006-99875.)
In the art disclosed in the above-mentioned publication, the position of the prism making up the polarization beam splitter can be fixed accurately, however, a position is specified of the outside of the prism making up the polarization beam splitter. Therefore, if the polarization beam splitter itself is not manufactured accurately, a position of a polarization surface for controlling reflection and transmission can not be fixed at the accurate position, which is a problem.